The function of the four valves in the mammalian heart (including that of humans) is to enable forward propulsion of blood without regurgitation, or backwards flow. The valves sit in between the chambers of the heart and its great vessels. The left heart pumps oxygenated blood under high pressure to the systemic circulation, while the right heart pumps deoxygenated blood under lower pressure to the pulmonary circulation.
The left and right heart each have two valves: the atrioventricular valves, and the semilunar valves. In the left heart, these are the mitral and aortic valves, respectively. In the right heart, these are the tricuspid and pulmonic valves. The atrioventricular valves divide the atria (low pressure filling chambers) from the ventricles (muscular pumping chambers). The semilunar valves separate the ventricles from their outflow great vessels.
Focusing for now on the cardiac cycle of the left side of the heart, the normally functioning mitral (atrioventricular) valve opens to permit the left atrium to empty under low pressure into the relaxed left ventricle during diastole. At the onset of ventricular systole, rising pressure in the left ventricle closes the mitral valve, so that blood does not flow back into the left atrium. When left ventricular pressure rises to exceed that in the aorta, the aortic (semilunar) valve opens to permit left ventricular ejection of blood into the aorta. When the left ventricle has completed its ejection phase, and begins to relax, the aortic valve falls closed, preventing blood from regurgitating into the left ventricle. During isovolumic relaxation, the ventricular pressure falls with both aortic and mitral valves closed. Then the mitral valve opens allowing left atrial empyting again into the low pressure left ventricle, and pressure-volume cycling begins again. A similar sequence takes place in the right heart with its tricuspid and pulmonic valves, right atrium and ventricle, and pulmonary artery.
The cardiac valves in humans may be affected by a variety of congenital and acquired disorders. The functional result of these disorders may include:
1. Valvular stenosis, whereby a failure of the valve to open completely causes increased resistance to blood flow across that valve.
2. Valvular regurgitation, whereby failure of the valve (or associated structures) to close completely permits blood to leak backwards into the normally protected chamber.
The consequences of valvular stenosis and regurgitation produce major human disease, including congestive heart failure, adverse remolding of the cardiac chambers, disabling symptoms, heart rhythm disturbances, decreased functional capacity, and death. For these reasons, medical science includes treatments to repair, to replace, or to supplement abnormally functioning heart valves.
One strategy to treat malfunctioning heart valves is to implant into the heart a prosthetic valve which supplements or replaces the functions of the diseased valve. For example, the transcatheter implantation of a prosthetic valve into the position of the aortic, or the pulmonic valve (the semilunar valves) has been used successfully to treat both stenosis and regurgitation of these valves. In many such applications, the transcatheter valve is implanted without removing the diseased or malfunctioning semilunar valve; in this way the malfunctioning valve's tissue is excluded from the main path of blood flow, and its function is partially or wholly replaced by the new valve. In addition to the treatment of diseased native valves, prosthetic or transplanted heart valves can also be treated in this fashion.
The structure and function of the semilunar valves differ importantly from those of the atrioventricular valves. Features which are significant for purposes of the present disclosure include:
1. The circular or mildly elliptical conformation of tissue, with low distensibility, surrounding the semilunar valves.
2. The tubular nature of the ventricular outflow tract (below the semilunar valves) and of the great vessels (above the semilunar valves).
3. The higher velocity and pressurized nature of flow across the semilunar valves compared to the atrioventricular valves.
4. The complex three-dimensional structure of the atrioventricular valves.
For these and other reasons, there is a need in the art for methods and devices that utilize certain aspects of the semilunar valve complex and its surrounding tissues for like purposes of anchoring, aligning, stabilizing, fixing, or otherwise enabling the implantation of prosthetic devices into the atrioventricular valve and elsewhere in the heart or in the vicinity thereof.